ABSTRACT MST1 and MST2 are the mammalian Ste20-related protein kinases most closely related to Drosophila Hippo, a major regulator of cell proliferation and survival during development. Overexpression of MST1 or MST2 in mammalian cells is proapototic; however, little is known concerning the physiologic regulation of the endogenous MST1/MST2 kinases, their role in mammalian cell proliferation, or the identity of the MST1/MST2 substrates critical to proliferative regulation.
We show that MST1 and MST2 activity increases during mitosis, especially in nocodazole-arrested mitotic cells, where these kinases exhibit both an increase in both abundance and activation. MST1 and MST2 also can be activated nonphysiologically by okadaic acid or H2O2. The MOBKL1A and MOBKL1B polypeptides, homologs of the Drosophila MATS polypeptide, are identified as preferred MST1/MST2 substrates in vitro and are phosphorylated in cells in an MST1/MST2-dependent manner in mitosis and in response to okadaic acid or H2O2. MST1/MST2-catalyzed MOBKL1A/MOBKL1B phosphorylation alters the ability of MOBKL1A/MOBKL1B to bind and regulate downstream targets such as the NDR-family protein kinases. Thus, MOBKL1A/MOBKL1B phosphorylation in cells promotes MOBKL1A/MOBKL1B binding to the LATS1 kinase and enables H2O2-stimulated LATS1 activation loop phosphorylation. Most importantly, replacement of endogenous MOBKL1A/MOBKL1B by a nonphosphorylatable mutant is sufficient to accelerate cell proliferation substantially by speeding progression through G1/S as well as mitotic exit.
These results establish that MST1 and MST2 are activated in mitosis and catalyze the mitotic phosphorylation of MOBKL1A/MOBKL1B. MOBKL1A/MOBKL1B phosphorylation, in turn, is sufficient to inhibit proliferation through actions at several points in the cell cycle.

[Show abstract][Hide abstract]ABSTRACT:
Primary cilia are microtubule-based organelles that protrude from polarized epithelial cells. Although many structural and trafficking molecules that regulate ciliogenesis have been discovered, signalling proteins are not well defined. Here we show that the MST1/2-SAV1 complex, a core component of the Hippo pathway, promotes ciliogenesis. MST1 is activated during ciliogenesis and localizes to the basal body of cilia. Depletion of MST1/2 or SAV1 impairs ciliogenesis in cultured cells and induces ciliopathy phenotypes in zebrafish. MST1/2-SAV1 regulates ciliogenesis through two independent mechanisms: MST1/2 binds and phosphorylates Aurora kinase A (AURKA), leading to dissociation of the AURKA/HDAC6 cilia-disassembly complex; and MST1/2-SAV1 associates with the NPHP transition-zone complex, promoting ciliary localization of multiple ciliary cargoes. Our results suggest that components of the Hippo pathway contribute to establish a polarized cell structure in addition to regulating proliferation.

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